1. Field of the Invention
The present invention relates to a process for cleaning a surface of a thin film of oxide superconductor.
The process according to the present invention is used for preparing a layered structure containing at least one thin film of oxide superconductor, more particularly, for depositing more than two thin films of oxide superconductor each possessing a different crystal orientation successively on a substrate and for depositing, on a thin film of oxide superconductor, another thin film of different material.
The process according to the present invention is used also for removing photo-resist from a thin film of oxide superconductor.
2. Description of the Related Art
Oxide superconductors are expected to be used in a variety of applications due to their higher critical temperatures than conventional metal superconductors. In fact, Y--Ba--Cu--O oxide superconductor possesses the critical temperature above 80 K. and Bi--Sr--Ca--Cu--O and Tl--Ba--Ca--Cu--O oxide superconductors possesses that of above 100 K.
These oxide sutterconductors, however, possess crystal anisotropy in their superconducting properties. In fact, the highest critical current density is observed in a direction which is perpendicular to c-axis of their crystal. From this fact, the direction of crystal must be considered in actual utilization of these oxide superconductors.
When the oxide superconductors are used in superconducting electronics applications such as superconducting devices or integrated superconducting circuits, it is indispensable to prepare at least one thin film of the oxide superconductor and to laminate a plurality of thin films. The problem of crystal anisotropy becomes much server in such superconducting devices or integrated superconducting circuits. For instance, in order to realize high-performance superconducting devices or integrated superconducting circuits, it is requested to prepare two kinds of superconducting wiring lines: one part in which electric current flows in parallel with a surface of substrate and another part in which electric current flows perpendicularly to the surface of substrate. For example, in superconducting electrodes, current flows in parallel with the surface of substrate while, in interlayer connecting superconducting wiring lines which connect layers stratified on the substrate, current flows perpendicularly to the surface of substrate. Therefor, when oxide superconductor is used in high-performance superconducting devices or integrated superconducting circuits, it is requested to deposit both of c-axis orientated thin film of oxide superconductor in which the critical current density along the direction which is in parallel with the surface of substrate is higher than the other directions and of a-axis (or b-axis) orientated thin film of oxide superconductor in which the critical current density along the direction which is perpendicular to the surface of substrate is higher than the c-axis orientated thin film on a common surface of a substrate. Hereinafter, only a-axis oriented thin film is referred, since electric current flows equally along the direction which is perpendicular to the surface of substrate in a-axis orientated thin film and in b-axis orientated thin film.
In the multi-layered structures for the superconducting devices or integrated superconducting circuits, two layers of a c-axis oriented thin film of oxide superconductor and of an a-axis oriented thin film of oxide superconductor must be deposited successively. Crystal orientation of the thin film of oxide superconductor can be controlled by selecting or adjusting film-forming temperature which is determined by substrate temperature. In fact, the a-axis oriented thin film can be realized at a substrate temperature which is lower by about 50.degree. to 100.degree. C. than a substrate temperature at which the c-axis oriented thin film grows.
In a superconducting junction of so-called Josephson Junction realized with oxide superconductor, it is requested to deposit a bottom superconductor layer, an intermediate thin film of non-superconductor and a top superconductor layer on a substrate successively in this order.
Josephson element is a two-terminals element, so that a logical circuit consisting of Josephson elements alone becomes complicated. In order to overcome this demerit of complexity, a variety of ideas of three-terminals elements are proposed. In the superconductor transistors consisting of superconductor and semiconductor which is a typical three-terminals element, it is also required to combine a thin film of semiconductor with a thin film of oxide superconductor and hence successive deposition of thin films each made of different material is required.
In these superconducting elements, a superconducting current passes through a thin film of non-superconductor sandwiched between two adjacent layers of superconductors positioned close to each other. A distance between the two superconductors is determined by the coherence length of superconductor. In the case of oxide superconductor, the distance between two superconductors must be several nanometer because its coherence length is very short.
On the other hand, from a point of view as performance of the superconducting devices, all thin films in the superconducting device must have high crystallinity, in other words, these thin films are preferably made of a single crystal or polycrystal having crystal orientation which is similar to single crystal. When the superconducting device has thin film(s) made of polycrystal whose crystal orientation is not well-ordered or has amorphous thin film(s), high-performance of the superconducting devices can not be expected and hence function thereof become unstable.
When more than two thin films are deposited successively on a common substrate, it is usual practice to subject a surface of a bottom superconductor layer to cleaning operation before a top superconductor layer is deposited, otherwise electrical continuity between the bottom superconductor layer and the top superconductor layer is spoiled due to contaminants adsorbed on a surface of the bottom superconductor layer or undesirable oxides produced on the surface. Discontinuity of two layers result in formation of an undesirable junction between two layers. Superconducting devices or integrated superconducting circuits having such undesirable junction do not show desired performance and sometimes do not work.
In particular, the surface condition of the bottom superconductor layer should be considered carefully because the coherence length of oxide superconductors is very short. Still more, oxygen of oxide superconductors is rather unstable and easily escape out of the thin film. Excessive oxygen deficient result in deterioration of superconducting properties and, in the worst case, loss of superconductivity.
Therefore, the surface of bottom superconductor layer must be clean and also must have well-ordered crystallinity or superconducting property.
In the field of semiconductor industries, surfaces are cleaned with ultra-pure water, by chemical washing, dry or wet etching or the like. In the case of oxide superconductors, however, these clearing technique can not be used due to high reactivity of oxide superconductors. If the surface of thin film of oxide superconductor is treated by these known techniques, undesirable reaction occur on the surface, resulting in that cleanness of the surface become worse and crystallinity and superconducting property are lost.
It is also known to deposit the top superconductor layer, just after the bottom superconductor layer of oxide superconductor has been deposited, in an identical chamber. This technique, however, requires a big chamber and materials to be used for the top superconductor layer are limited.
Therefore, an object of the present invention is to solve the problems and to provide an improved process for preparing a layered structure containing at least one thin film of oxide superconductor without deteriorating superconducting properties of the thin film of oxide superconductor.
Another object of the present invention is to provide an improved process for depositing more than two thin films of oxide superconductor each possessing a different crystal orientation successively on a substrate.
Still another object of the present invention is to provide an improved process for depositing, on a thin film of oxide superconductor, another thin film of different material.
The process according to the present invention is applicable also to remove photo-resist residue which is remained on a thin film of oxide superconductor after pattering operation.
Namely, pattering of a thin film of oxide superconductor can be effected by a variety of techniques. The most popular pattering technique is to protect a predetermined surface area on the thin film of oxide superconductor with photo-resist followed by etching. In the case of pattering of oxide superconductors which are chemically active, wet-etching is not suitable and hence dry-etching such as reactive ion etching, electron beam etching or argon ion milling is used.
Generally, pattering of thin films with photo-resist is carried out as following. At first, a surface of the thin film is cleaned and then photo-resist is coated thereon. The photo-resist is cured by pre-baking so that the photo-resist is fix to the thin film. The pre-baked photo-resist is irradiated with light through a mask. After development, the irradiated photo-resist is subject to post-baking and then is etched. Finally, remaining photo-resist is removed by wet-treatment with photo-resist remover or oxidizing reagent to clean the surface of the thin film.
Pattering of thin film of oxide superconductor is carried out almost same sequence as above. In the case of thin film of oxide superconductor, however, serious deterioration of superconductivity is often observed after the pattering. The present inventors found such a fact that such deterioration of superconductivity of the thin film is caused by chemical reaction during removal of the photo-resist. In fact, because of relatively high reactivity, the oxide superconductor react with liquid photo-resist remover or oxidizing reagent at the removing stage of photo-resist, resulting in that a surface of the thin film is roughened and that composition of oxide superconductor is changed. Another thin film deposited on such deteriorated thin film does not show desired superconducting properties and hence it is difficult to obtain a layered structure of high-performance.
It is also known to remove photo-resist by dry-process so-called "ashing" technique in which photo-resist is burnt in oxygen plasma. This ashing technique, however, reduce oxide superconductor during burning of photo-resist, resulting in that oxygen in oxide superconductor is lost. Oxygen-deficient oxide superconductor deviated from desired stoichiometry and lose sharply the superconductivity and often become non-superconductor.
At the today's level of pattering technology, use of photo-resist is indispensable. This situation is same in the thin film of oxide superconductor.
Therefore, still another object of the present invention is to solve the problems and to provide a process for removing photo-resist residue remained on a thin film of oxide superconductor.